The field of the invention relates to sheet presses for printing individual documents or other articles.
Two types of printing presses are generally well known to the printing industry, these being web presses and sheet presses. The former is used primarily for continuously printing a roll or web of paper or other substrate. The web is subsequently cut to desired lengths. The latter type of press is used for printing individual, pre-cut documents, envelopes or other articles.
A sheet press generally comprises one or more plate cylinders, a blanket cylinder, and an impression cylinder. One of the plate cylinders is typically driven by a power source. A gear drive is provided for coupling these two elements. Ink is provided to the etched plate mounted to the outer surface of the plate cylinder. This ink is transferred by the engagement of the outer surfaces of the plate and blanket cylinders, the latter having an outside surface defined by ink receptive blanket material The paper to be printed passes between the blanket and impression cylinders. A precise gearing arrangement maintains the relative positions of the plate and blanket cylinders when the press is in operation.
A problem occurs in sheet presses when a document is not fed and the plate and blanket cylinders must be separated. Such separation, known as the "off impression" mode, is necessary as too much ink will accumulate on the blanket cylinder if the press is allowed to keep running at this time. However, separation of the cylinders is only accomplished through separation of the gear train which joins these two elements.
Another drawback of presently known sheet presses is the inability to advance or retard the press while it is running. This is due to the fact that the gearing must be so precise that play that would permit such adjustment cannot be allowed. Devices known as indexers are accordingly employed to advance or regard the press while in it is in the static mode. A full 360.degree. of adjustment is possible by using such indexers. Another alternative is to employ a pair of helical gears which are axially displaced with respect to each other. A major disadvantage of this approach is that the plate cylinder can only be displaced by a very limited amount.